Control of pollen fertility is essential in hybrid crop production. See, for example, J. M. Poehlman, BREEDING FIELD CROPS, 3rd ed. (Van Nostrand and Reinhold, New York, N.Y., 1987), which is herein incorporated by reference. In hybrid maize production, for example, control of pollen fertility is typically accomplished by physically removing the male inflorescence, or tassel, prior to pollen shed. Manual detasseling is highly labor intensive. Although mechanical detasseling is less labor intensive than manual detasseling, it is less reliable and requires subsequent examination of the crop and commonly, remedial manual detasseling. Both methods of detasseling cause a loss of female parent yield.
Most major crop plants of interest, however, have both functional male and female organs within the same flower; therefore, emasculation is not a simple procedure. While it is possible to remove by hand the pollen-forming organs before pollen is shed, this form of hybrid production is extremely labor intensive and expensive.
Pollen fertility also can be controlled by applying a foliar spray having male gametocidal properties. Cross et al., Sex Plant Reprod. 4: 235 (1991). For example, the application of ethrel to anthers results in additional pollen mitoses in wheat, degeneration of tapetal cells in barley, and malformed or aborted pollen in Eragrostis. Bennet et al., Nature 240: 566 (1972), Colhoun et al., Plant Cell Environ. 6: 21 (1983); Berthe et al., Crop Sci. 18: 35 (1978). However, the chemical approach is labor intensive, presents potential problems with the toxicity of chemicals introduced into the environment, and can be very sensitive to the timing of application.
In addition, commercial production of hybrid seed by use of gametocides is limited by the expense and availability of the chemicals, as well as by the reliability and length of action of the applications. A serious limitation of gametocides is that they have phytotoxic effects, the severity of which are dependent on genotype. Other limitations include that these chemicals may not be effective for crops with an extended flowering period because new flowers produced may not be affected. Consequently, repeated application of chemicals is required.
Many current commercial hybrid seed production systems for field crops rely on a genetic means of pollination control. Plants that are used as females either fail to shed pollen, produce pollen that is biochemically unable to effect self-fertilization, or fail to make pollen. Plants that are unable to self-fertilize are said to be "self-incompatible." Difficulties associated with the use of a self-incompatibility system include availability and propagation of the self-incompatible female line, and stability of the self-compatibility. In some instances, self-incompatibility can be overcome chemically, or immature buds can be pollinated by hand before the bio-chemical mechanism that blocks pollen is activated. Self-incompatible systems that can be deactivated are often very vulnerable to stressful climatic conditions that break or reduce the effectiveness of the biochemical block to self-pollination.
Of more widespread interest for commercial seed production are genetic pollen-control-based systems causing male sterility. These systems are of two general types: (1) nuclear male sterility, which is the failure of pollen formation because of mutations in one or more nuclear genes or (2) cytoplasmic-genetic male sterility, commonly referred to as "cytoplasmic male sterility" (CMS), in which pollen formation is blocked or aborted because of an alteration in a cytoplasmic organelle, which generally is the mitochondria.
Nuclear sterility can be either dominant or recessive. Typically, dominant sterility can only be used for hybrid seed formation if vegetative or clonal propagation of the female line is possible. Recessive sterility can be used if sterile and fertile plants are easily discriminated. Commercial utility of dominant and recessive sterility systems is limited, however, by the expense of clonal propagation and rouging the female rows of self-fertile plants, respectively.
Although there are hybridization schemes involving the use of CMS, there are limitations to its commercial value. One example of a CMS system is a specific mutation in the cytoplasmically located mitochondria which can, when in the proper nuclear background, lead to the failure of mature pollen formation. In some instances, the nuclear background can compensate for the cytoplasmic mutation and normal pollen formation occurs. Specific nuclear "restorer genes" allow pollen formation in plants with CMS mitochondria. Generally, the use of CMS for commercial seed production involves the use of three breeding lines: a male-sterile line (female parent), a maintainer line which is isogenic to the male-sterile line but contains fully functional mitochondria, and a male parent line. The male parent line may or may not carry the specific restorer genes in the cytoplasm.
For crops such as vegetables for which seed recovery from the hybrid is unimportant, a CMS system can be used without restoration. For crops for which the fruit or seed of the hybrid is the commercial product, the fertility of the hybrid seed must be restored by specific restorer genes in the male parent or the male-sterile hybrid must be pollinated. Pollination of non-restored hybrids can be achieved by including a small percentage of male fertile plants to effect pollination. In most species, the CMS trait is inherited maternally, since all cytoplasmic organelles are usually inherited from the egg cell only.
CMS systems possess limitations that can preclude them as a sole solution to production of male sterile plants. For example, one particular CMS type in maize (T-cytoplasm) confers sensitivity to the toxin produced by infection by a particular fungus. Although still used for a number of crops, CMS systems may break down under certain environmental conditions.
It is apparent therefore that a means to control pollen production by other than manual, mechanical, chemical and traditional genetic methods is greatly to be desired.